1. Field of the Invention
The present invention relates to a receiving apparatus for controlling a phase of a reception signal using a synchronization signal included in the reception signal.
2. Description of Related Art
To send and receive framed or packetized data between a sending and a receiving apparatuses, a correlation method is used to establish a frame synchronization of a reception signal in the receiving apparatus (see Japanese Unexamined Patent Application Publication No. 2005-176184). In the correlation method, when the sending apparatus sends an information signal to the receiving apparatus, a reference signal is added to the beginning of the information signal. The reference signal is a signal sequence having a specified signal pattern repeated periodically. Such reference signal is hereinafter referred to as a preamble. The receiving apparatus calculates a cross-correlation value between a signal pattern of a known preamble and a reception signal or an auto-correlation value of the reception signal to detect a timing (hereinafter referred to as a correlation peak position), which the calculated cross-correlation value or the auto-correlation value reaches its peak. Further, the receiving apparatus identifies a start timing of an information signal by evaluating a repetition cycle of the detected correlation peak position or a change in the repetition cycle thereof. It is possible to perform receiving processes including a demodulation of an information signal by identifying the start timing of the information signal, specifically by establishing a frame synchronization.
For example in a wireless LAN that is compliant with the IEEE 802.11a standard, a preamble called a short symbol is added to the beginning of an OFDM signal being sent and received. A receiving apparatus in the wireless LAN identifies a start timing of the OFDM signal using the short symbol, then demodulates the information signal according to the identified start timing. Then the receiving apparatus demodulates the information signal by a high-speed Fourier transformation based on the identified start timing.
An establishment of a frame synchronization by the correlation method as described in the foregoing is applied to a receiving apparatus of UWB (Ultra Wide Band), which is a short distance wireless communication standard, other than a wireless LAN. One of the UWB communication systems is standardized as ECMA-368 by the ECMA (European Computer Manufacturer Association), which is a standardizing organization. In this standard, the MB-OFDM (Multi-band Orthogonal Frequency Division Multiplexing) is employed to a PHY layer.
A configuration example of a synchronization timing apparatus for detecting a correlation peak position is shown in FIG. 4. A synchronization timing apparatus 41 of FIG. 4 is used in a MB-OFDM receiving apparatus. An I component signal ID(t) and a Q component signal QD(t), which are discrete baseband signals that are sampled and quantized by A/D converters (ADC) 40a and 40b are input to the synchronization timing detecting apparatus 41.
A correlator 411a inputs the I component signal ID(t) and a signal pattern r(i) of a preamble that is stored to a ROM (Read Only Memory) 412a so as to calculate a cross-correlation value CI(t) between ID(t) and r(i) Similarly a correlator 411b inputs the Q component signal QD(t) and a signal pattern r(i) of a preamble stored to a ROM (Read Only Memory) 412b so as to calculate a cross-correlation value CQ(t) between QD(t) and r(i). The cross-correlation values QD(t) and CQ(t) can be defined respectively by the following formulas (1) and (2).
                                          C            I                    ⁡                      (            t            )                          =                              1            A                    ⁢                                    ∑                              i                =                0                                            P                -                1                                      ⁢                                                            I                  D                                ⁡                                  (                                      t                    +                                          i                      ×                      m                                                        )                                            ⁢                                                          ⁢                              r                ⁡                                  (                  i                  )                                                                                        (        1        )                                                      C            Q                    ⁡                      (            t            )                          =                              1            A                    ⁢                                    ∑                              i                =                0                                            P                -                1                                      ⁢                                                            Q                  D                                ⁡                                  (                                      t                    +                                          i                      ×                      m                                                        )                                            ⁢                                                          ⁢                              r                ⁡                                  (                  i                  )                                                                                        (        2        )                                A        =                              ∑                          i              =              0                                      P              -              1                                ⁢                                                                  r                ⁡                                  (                  i                  )                                                                    2                                              (        3        )            
In the formulas (1) and (2), P refers to the number of samples for one repetition pattern in a preamble. For a PLCP (Physical Layer Convergence Protocol) preamble defined by the abovementioned UWB standard, for example, P=165. In the above formulas, m refers to an integer of 1 or more that indicates an oversampling rate of the ADCs 40a and 40b. Further, A refers a normalization constant that is defined by the formula (3).
A sum of squares calculator 413 inputs cross-correlation values CI(t) and CQ(t) that are calculated by the correlators 411a and 411b, and calculates a sum of squares thereof as in P(t)=CI(t)2+CQ(t)2.
A threshold evaluator 414 evaluates a threshold for the sum of squares P(t) that is output from the sum of squares calculator 413 to detect a correlation peak position and outputs the detected correlation peak position as a symbol timing. A principle of the threshold evaluation by the threshold evaluator 414 is described hereinafter in detail with reference to FIG. 5. In FIG. 5, PSn(n=1, 2, . . . , 21) indicates a Packet Sync Sequence that constitutes a PLCP preamble. In the PLCP preamble, the packet sync sequence is repeated for 21 symbols.
When a symbol pattern of the packet sync sequence is applied to the formulas (1) and (2) to calculate the cross-correlation values CI(t) and CQ(t), and the sum of squares P(t) thereof, the sum of squares P(t) of the cross-correlation values will reach a peak at a timing when the reception signal ID(t) and QD(t) correspond with the symbol pattern r(i) of the packet sync sequence. The timing when the reception signals ID(t) and QD(t) correspond with the symbol pattern r(i) of the packet sync sequence is a timing when calculating a cross-correlation for the symbol pattern r(i) of the packet sync sequence using a set of sampling points for the reception signals ID(t) and QD(t) which have a delimiter position of the packet sync sequence at the beginning thereof. Specifically, at delimiter positions T1, T2, and T3 of the packet sync sequence shown in FIG. 5, a correlation peak is observed in P(t). A threshold of P(t) is evaluated by the threshold evaluator 414 and the correlation peak positions including T1, T2 and T3 shown in FIG. 5 are output as symbol timings. By using the symbol timings, it is possible to establish a symbol and a frame synchronization, thereby enabling to do a demodulation process etc for a subsequent OFDM signal.
As described in the foregoing, when the reception signals ID(t) and QD(t), and the preamble r(i) that are input to the correlators 411a and 411b correspond, a correlation peak can be observed in the cross-correlation values CI(t) and CQ(t), and the sum of squares P(t) thereof. An accurate synchronization can be established by accurately capturing the correlation peak. To accurately capture the correlation peak, a sampling rate for the reception signals ID(t) and QD(t) in the ADCs 40a and 40b needs to be higher, and intervals for the sampling points to calculate correlations by the correlators 411a and 411b needs to be narrower.
A problem generated when the sampling rate for the reception signal is low described hereinafter in detail. FIGS. 6A and 6B show the cross-correlation value CI(t) that is computed using a discrete I component signal obtained by the ADC 40a when the sampling rate of the ADC 41a is two times higher than the maximum frequency included in the baseband signal I(t). The curve R indicated by dotted lines in FIGS. 6A and 6B indicates a cross-correlation function between a continuous signal I(t) and a preamble when the ADC 40a does not perform a sampling. Further, the horizontal axis in FIGS. 6A and 6B indicates time that is standardized by a sampling time Ts (an inverse of the sampling rate), where the time 0 is a correlation peak position.
If a sampling phase of the reception signal I(t) in the ADC 40a is a phase that is possible to sample a delimiter position of a packet sync sequence PSn, the discrete cross-correlation value CI(t) that is output by the correlator 411a will be the one as in FIG. 6B. In FIG. 6B, with a center of a time k, four sampling points before and after the time k are indicated by cross-correlation values CI(k−4) to CI(k+4). In the sampling phase like the one shown in FIG. 6B, a correlation peak is included in the cross-correlation values output from the correlator 411a, thus an accurate establishment of a synchronization can be possible by detecting the correlation peak. In FIG. 6B, CI(k) corresponds to the correlation peak.
On the other hand in FIG. 6A, cross-correlation values CI(k−4) to CI(k+4) that are output from the correlator 411a are shown when the sampling time of the reception signal I(t) in the ADC 40a is shifted for a ½ period as compared to the case as in FIG. 6B where the sampling time of the reception signal I(t). In such sampling phase, a genuine correlation peak is not included in the cross-correlation values that are output from the correlator 411a. Accordingly in such sampling phase, a correlation peak value cannot be accurately detected and it is difficult to detect a correlation peak depending on a threshold that is specified to the threshold evaluator 414. Furthermore, if a detection threshold for a correlation peak in the threshold evaluator 414 is set smaller in order to detect a small correlation peak, it is more likely to incorrectly detect a peak in a cross-correlation value that is generated due to noise.
Therefore, to prevent a fluctuation of a synchronization accuracy that is dependent on a sampling phase of the ADC 40a, it is necessary to improve the sampling rate of the ADC 40a to conduct an oversampling.
As described in the foregoing, to accurately establish a synchronization by the correlation method in the receiving apparatus for receiving framed (packetized) data, it is necessary to conduct an oversampling when sampling and discretizing a reception signal. However it has now been discovered that an increase in a sampling rate causes an increase in a circuit size of the receiving apparatus and also an increase in power consumption. Therefore, to reduce the circuit size of the receiving apparatus and the power consumption, it is desirable to reduce the sampling rate of the reception signal while maintaining an accuracy of a synchronization establishment.